Based on magnetohydrodynamic theory a shell model is developed to describe debris motion of high-altitude nuclear explosion. The debris motion parameter of Starfish is simulated. The model is verified by comparison with the results available from the literature. For the comparison between typical high-yield and low-yield nuclear explosion, the debris motions of kiloton and megaton at 100 km, 400 km and 1500 km are especially simulated. The difference in expansion law, caused by explosion yield, is analyzed. The results show that explosion condition and atmospheric environment have a significant influence on the expansion of the law of the debris cloud, and the debris environmental parameters in different directions are significantly different from each other.